In any data or communication signal, interference between symbols in the signal is a contributor to both vertical and horizontal signal variations. The basic effect is that the shape and timing of subsequent data-symbols is affected by (or has a memory of) the precise sequence of prior symbols. That is, in the particular application of the display of an eye diagram on an oscilloscope, the symbols immediately preceding an observed bit-interval affect an eye-diagram for the observed bit-interval for a random bit stream.
Therefore, it would be beneficial to account for such interference when processing data communication signals, and when viewing an eye diagram on an oscilloscope.